Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular constituents. To characterize a composition in a remote or hostile environment, optical fibers may advantageously be used to deliver excitation energy to a sample under investigation and to carry scattered radiation back to means for spectral analysis. An excitation source path may take the form of a laser providing a stimulus at an appropriate wavelength coupled to an input fiber, and a collection path may comprise one or more additional fibers carrying return radiative information to a spectral analysis tool such as a spectrograph.
Fiber-optic probes make it possible to collect optical information such as Raman spectra without having to place the material being characterized inside a spectrometer housing. Such probes therefore simplify the interfacing of spectroscopic systems to chemical processes, and allow analytical instruments to be located remotely from hostile environments in need of spectroscopic monitoring.
The first remote fiber optic probes for Raman spectroscopy, reported by the McCreery group in the early 1980's, used a single optical fiber to deliver laser light to the sample and a single optical fiber to collect light scattered by the sample. This dual-fiber approach offered important benefits. For one, the probe could be made less than one millimeter in diameter, making Raman measurements possible for samples with limited accessibility. In addition, the probe could be placed directly into a hostile sample, since only silica and the encapsulation material were exposed.
More modern imaging probes, however, utilize multiple optical components at or near the distal ends of the excitation and collection fibers. As described in commonly assigned U.S. Pat. No. 5,377,004, entitled REMOTE OPTICAL MEASUREMENT PROBE, merging of the excitation radiation into a combined excitation/collection path preferably takes place proximate to the remote end of the fibers, requiring a beam combiner to be located near the sample. In addition, to prevent laser wavelengths from entering into the collection fiber(s), one or more rejection filters are preferably situated in the probehead as well.
Increasingly, imaging probes of the type just described are being utilized to monitor processes involving hazardous chemicals. Because of the hazardous nature of the materials, it is necessary to ensure that insertion of the probes into pipes or vessels will not leak into the surrounding environment. However, due to the need for additional components in the optical paths from the sample to the tips of the excitation and collection fibers in an imaging probe, leak paths may be created along the various optical paths unless sufficient safeguards are provided.